1. Field of the Invention
The present invention relates to a superconductor device and particularly to a superconductor device having a pair of electrodes made of Bi-system oxide superconductor materials including at least one alkaline earth metal and a barrier layer between the pair of electrodes.
2. Description of the Related Art
Recently, there have been found certain oxide superconductors having a Tc higher than the temperature of liquid nitrogen and, thereby, applicability of the superconductor has been widened greatly.
As an application of the oxide superconductor, there have been proposed Josephson devices of various types such as the point contact type wherein two oxide superconductors are contacted pointwise, bridge type wherein two oxide superconductor thin films are connected with a micro thin film bridge and bonding type wherein two oxide superconductor thin films are connected with a bonding line made of a noble metal such as Au, Ag or the like.
Among those Josephson devices, the point-contact type exhibits only a poor reproducibility and is extremely unstable in operation. The Josephson device of bridge type mentioned above has a disadvantage in that it is easily damaged by a slight electrostatic shock.
In order to avoid these disadvantages of the conventional Josephson devices, an oxide superconductor device having a structure of junction type is highly desired. However, in order to realize this, there are some problems to be solved as follows.
Upon forming a thin film of an oxide superconductor, a very high temperature, higher than about 600.degree. C. is needed and, thereby, the material forming the barrier layer disintegrated by diffusion upon forming a counter electrode of an oxide superconductor. Also, since the thermal expansion coefficients of the material forming the electrode layer and that forming the barrier layer are different from each other, thermal stresses are caused in respective layers when cooled to the ambient temperature and, thereby, the superconductivity of the electrode layer is greatly deteriorated and/or pin holes are generated in the barrier layer.
Further, the crystallinity of the counter electrode is made inferior to that of the base electrode due to mismatch between the lattice constants of materials used for the counter electrode and the barrier layer.
C. T. Rogers et al. proposed heteroepitaxial YBa.sub.2 Cu.sub.3 O.sub.7-x --PrBa.sub.2 Cu.sub.3 O.sub.7-x --YBa.sub.2 Cu.sub.3 O.sub.7-x Josephson devices of junction type (See Appl. Phys. Lett. 55(19), 6 Nov. 1989). However, the Y-system oxide superconductor used therein has a disadvantage in that it exhibits a large time dependent deterioration by absorbing water vapor.